1. Field of the Invention
The present invention relates to a communication system that reproduces voice in real-time at plural terminal equipments, a server equipment and terminal equipments that are used in the system.
2. Description of the Related Art
In recent years, a voice multicast system that communicates voice simultaneously and in real-time at many locations has been spreading. Applications of such a voice multicast system include a Push to talk over cellular (PoC) service that enables conversation among three or more cellular phones, and a TV meeting system for having a meeting among people being far away from one another. In a voice multicast system like this, voice can be reproduced simultaneously at multiple terminal equipments by receiving voice originated from one terminal equipment at a server equipment and then by transferring the received voice from the server equipment to other terminal equipments. In addition, voice is transmitted and received by being digitalized and divided into packets per certain data unit. By saving a communication procedure such as an error handling, priority is placed upon real-time communication to reliability of communication. According to a voice multicast system like this, information can be conveyed to one another among multiple people staying at distant locations by saving time and moving costs.
However, in a voice multicast system, there may be situations where the amount of packets processed at a server equipment temporarily increases, for example, in the evening or at New Year's; when a user moves with a cellular phone to a place where a radio wave condition is poor; and when many users use cellular phones in areas covered by one base station and thus band allotment for each cellular phone becomes less. Under such situations the network is not abnormal, yet it makes difficult for the network to conduct packet communication at certain intervals without losing any packet, which hinders reproducing voice in real-time.
There is one known technique to cope with such a problem that a buffer is provided at a terminal equipment and voice is reproduced after accumulating packets transferred from a server at the buffer for a certain period of time in order to absorb delay of packets or fluctuations of delay. In addition, Japanese Patent Application Laid-open No. 2005-136742 describes a technique that changes the amount of data accumulated in a buffer at each terminal equipment and timing of voice reproduction dynamically, in response to situations of packets arriving at the network (server equipment). According to a technique described in Japanese Patent Publication Laid-open No. 2005-136742, even when packets are delayed, voice can be reproduced without causing any discomfort for users.
Moreover, in a voice multicast system in which communication is conducted between many terminal equipments and a server equipment, there is another problem that detecting where on the network a fault occurs is difficult.
Regarding this point, Japanese Patent Application Laid-open No. 2003-37599 describes a technique that detects a fault on the network by a network management device and notifies an administrator of information about the fault. Japanese Patent Application Laid-open No. 11-234273 describes a technique that notifies predetermined multiple administrators of faults on the network.
However, for example, if packets are delayed over several hundreds msecs, or if packet loss occurs in succession, then the technique described in Japanese Patent Application Laid-open No. 2005-136742 cannot absorb them and thus inconveniences arise such as abrupt disconnection of sound or inaudibility of conversation due to too much missing of voice.
Furthermore, although the techniques described in Japanese Patent Application Publication Laid-open Nos. 2003-37599 and 11-234273 can detect communication fault of the network, they cannot detect fault such as the above-described packet loss that is not abnormal on the network. Because of this, a user utilizing each terminal equipment cannot confirm whose terminal equipment is experiencing fault among multiple users who are engaged in conversation, and therefore, there are problems that the user is required to repeat talks or only one person is left out of topics or the like.